Electrochemical energy storage systems, such as batteries, supercapacitors and the like, have been widely proposed for large-scale energy storage applications. Various battery designs, including flow batteries, have been considered for this purpose. Compared to other types of electrochemical energy storage systems, flow batteries can be advantageous, particularly for large-scale applications, due to their ability to decouple the parameters of power density and energy density from one another.
Flow batteries generally include negative and positive active materials in corresponding electrolyte solutions, which are flowed separately across opposing faces of a membrane or separator in an electrochemical cell containing negative and positive electrodes. The flow battery is charged or discharged through electrochemical reactions of the active materials that occur inside the two half-cells. As used herein, the terms “active material,” “electroactive material,” “redox-active material” or variants thereof will synonymously refer to a material that undergoes a change in oxidation state during operation of a flow battery or like electrochemical energy storage system (i.e., during charging or discharging).
Although flow batteries hold significant promise for large-scale energy storage applications, they have often been plagued sub-optimal energy storage performance (e.g., round trip energy efficiency) and limited cycle life, among other factors. Despite significant investigational efforts, no commercially viable flow battery technologies have yet been developed. Certain issues leading to poor energy storage performance, limited cycle life, and other performance-degrading factors are discussed hereinafter.
Balanced oxidation and reduction of the active materials within each half-cell of a flow battery are desirable electrochemical reactions, since these reactions contribute to the flow battery's proper operation during charging and discharging cycles. Such reactions may be referred to herein as “productive reactions.” Undesirable parasitic reactions can also occur within one or both half-cells of flow batteries and related electrochemical systems. As used herein, the term “parasitic reaction” will refer to any side electrochemical reaction that takes place in conjunction with productive reactions.
Parasitic reactions can involve a component of an electrolyte solution that is not the active material. Electrochemical reactions of an active material that render the active material unable to undergo reversible oxidation and reduction can also be considered parasitic in nature. Parasitic reactions that commonly occur in aqueous electrolyte solutions are evolution of hydrogen and/or oxidation by oxygen. Hydrogen evolution, for example, can at least partially discharge the negative electrolyte of an electrochemical system while leaving the positive electrolyte unchanged. Even more significantly, parasitic reactions can change the pH of an electrolyte solution, which can destabilize the active material in some instances. Hydrogen evolution, for example, can raise the pH of an electrolyte solution by consuming protons. In non-aqueous electrolyte solutions, the electrolyte solvent can be similarly oxidized or reduced in an undesired parasitic reaction process. Further, in both aqueous and non-aqueous electrolyte solutions, electrode materials and other cell components can also undergo parasitic reactions (e.g., carbon or metal corrosion, separator oxidation, or the like) in some cases.
Discharge arising from parasitic reactions can decrease the operating efficiency and other performance parameters of flow batteries. In the case of a parasitic reaction that occurs preferentially in one half-cell over the other, an imbalance in state of charge can result between the negative and positive electrolyte solutions. The term “state of charge” (SOC) is a well understood electrochemical energy storage term that will refer herein to the relative amounts of reduced and oxidized species at an electrode within a given half-cell of an electrochemical system. Charge imbalance between the electrolyte solutions of a flow battery can lead to mass transport limitations at one of the electrodes, thereby lowering the round-trip operating efficiency. Since the charge imbalance can grow with each completed charge and discharge cycle, increasingly diminished performance of a flow battery can result due to parasitic reactions.
Charge rebalancing of one or both electrolyte solutions can be conducted to combat the effects of parasitic reactions. Various charge rebalancing strategies are available for electrolyte solutions, although they can be difficult to perform, costly, and time-consuming. For example, determining the true concentrations of oxidized and reduced species for the active material in an electrolyte solution can oftentimes itself be a difficult task, thereby further complicating the charge rebalancing process. Nevertheless, charge rebalancing can often be performed given sufficient diligence.
In contrast, the pH changes accompanying parasitic reactions can be significantly more difficult to address. Small changes in proton and hydroxide ion concentrations can produce dramatic swings in pH, which can be problematic for some active materials. Without adequate ways to address pH fluctuations, the working lifetimes of electrolyte solutions can be significantly compromised. Adjustment of pH through adding an extraneous acid or base to an electrolyte solution can be undesirable due to the accompanying changes in ionic strength and concentration of the active material. Further, addition of an extraneous acid or base at a rate sufficient to maintain a desired pH window in an electrolyte solution can sometimes be difficult, since the rates of parasitic reactions can often be highly variable. Since the pH changes resulting from parasitic reactions within electrolyte solutions can be additive, buffers may provide only temporary protection against pH changes until the buffering capacity has been exceeded.
In view of the foregoing, alternative rebalancing strategies for flow batteries and related electrochemical systems would be highly desirable in the art. The present disclosure satisfies the foregoing needs and provides related advantages as well.